Oxygen and hydrogen isotopic composition of the Bering Sea during the Last Glacial Maximum: constraints from pore water analyses

The North Pacific Intermediate Water (NPIW) is defined as the salinity minimum at water depths of 300-800 m and transports fresh water to low latitudes (Reid, 1965). The production of NPIW affects the circulation of the Pacific Ocean and has a potential contribution to global climate changes (Ohkushi et al. 2003; Tanaka et al. 2005; Cook et al. 2005). Hence investigating the paleoceanographic evolution in the formation regions of NPIW is critical for understanding the circulation of the Pacific Ocean and its role on global climate changes. So far, based on the microfossil assemblage such as foraminifera and radiolarians, it has been speculated that the NPIW was derived from the Bering Sea during the last glacial maximum (LGM), rather than the Okhotsk Sea as occurred at the present (Ohokushi et al., 2003; Tanaka et al., 2005). However the quantitative evidence (salinity, temperature, and isotopic composition of water etc.) directly indicating the past change of NPIW has not been observed yet. In this study, we reconstruct the distribution of oxygen and hydrogen isotopic compositions (δ18O and δD) of bottom water ranging from 700 m to 1000 m water depth during the LGM by using profile of δ18O and δD in pore water extracted from three piston cores retrieved from the depth transect at the Bering Sea shelf break: PC23 at 1000 m water depth; PC24 at 853 m water depth, and PC25 at 1157 m water depth. To assess changes in seawater isotopic compositions since the LGM, we use a mathematical model of the advective-diffusive transport of pore water in sediment. The fitting of model results to data indicates that the changes in δ18O of bottom water at the LGM with referenced to the present are 0.8 - 1.2 ± 0.1‰ at PC24 (853 m), 1.0 - 1.4‰ ± 0.1 at PC23 (1000 m), and 1.2 - 1.6‰ ± 0.1 at PC25 (1157 m), respectively, and those for δD are 3.5 - 4.5‰ ± 1.0 at PC24 (853 m), 4.0 - 5.5‰ ± 1.0 at PC23 (1000 m), and 7 - 9‰ ± 1‰ at PC25 (1157 m), respectively. The result clearly indicates that isotopic change is smaller in the core retrieved at the shallower water depth. The increase in isotopic composition during the LGM is primarily due to the increase in the global ice-sheet volume, which increases the isotopic composition. However the effect is uniform and the data cannot be fully explained. The depth-dependent component of isotopic composition change should be resulted from local salinity decrease depending upon the water depth, which also decreases the isotopic composition. The data suggests that low salinity water expanded as deep as 1000 m during the LGM and the magnitude of the salinity gradient at the sites was large compared to that at the present state. Today, NPIW is found at 200 - 400 m in the Bering Sea (Macdonald et al., 2001), while the water mass at 500-1,000 m is occupied by the saline Pacific Ocean water (Luchin et al., 1999) and as a result the salinity gradient is maintained small. Thus our result highly suggests that low salinity intermediate water sank into deeper depth during the LGM in the Bering Sea compared to the present situation.